IN A LETTER last month to President Carter, excerpts from which appear on the opposite page, city council member Marion Barry Jr. expressed an understandable disappointment that the elected local leadership of this city was excluded from a March 15 White House session on local affairs. As you may recall, we, too, voiced the view that this was a most unfortunate snub. As Mr. Barry goes on to state, it is the elected local leadership that must cope with the day-to-day problems of running the city - and that is directly answerable to the citizens.

Indeed, if a trip to a town meeting in Clinton, Mass., was supposed to embellish the President's understanding of local problems, a get-together with this community's leaders right here on the home turf might be still more enlightening. At least it would offer the President a different sense of what he ought to know or do about the District of Columbia than that provided by, say, House District Committee Chairman Charles C. Diggs Jr., who initiated last month's meeting.

In the meantime, we're still somewhat puzzled about precisely what is supposed to come from that session.So far, there is to be a task force on local affairs, headed by Vice President Mondale. The idea, as we got it, was for this task force to produce some sort of "package" of proposals to which the President could give support. That's fine - but it shouldn't take until the end of the year to materialize. On two occasions in this space, we've suggested a list of problems for a local agenda - including voting representation for the city in Congress, an increased federal payment, presidential support for Metro and additional measures of self-determination. Mr. Barry also offers specific proposals on these and other problems.

At any rate, the last thing this city needs is yet another semi-permanent layer of oversight through which all legislative proposals must pass before any action can be taken on them. The move for voting representation, for example, shouldn't be suspended this year just because the task force wants to include it in some "package" that isn't ready. Instead, President Carter and Vice President Mondale should meet with the city's leaders as soon as possible so that legislative proposals with administration support can begin flowing to Capitol Hill in an orderly fashion.